AbstractThis article illustrates evaluating the consequence of a large spill of sour water by modeling the plume of hydrogen sulfide and ammonia to toxic limits. This modeling uses chemical equilibrium constants and rigorous thermodynamic formulas, coupled with dispersion modeling to show how the plume contours of H2S and of NH3 vary with initial concentrations and pH. The results show that a pH adjustment is an effective mitigation measure. © 2007 American Institute of Chemical Engineers Process Saf Prog 27, 2007